ABSTRACT

The University of California Berkeley’s (UCB) Earthquake Alert Systems (ElarmS) is a network‐based earthquake early warning (EEW) algorithm that was one of the original algorithms developed for the U.S. west‐coast‐wide ShakeAlert EEW system. Here, we describe the latest update to the algorithm, ElarmS v.3.0 (ElarmS‐3 or E3). A new teleseismic filter has been developed for E3 that analyzes the frequency content of incoming signals to better differentiate between teleseismic and local earthquakes. A series of trigger filters, including amplitude‐based checks and a horizontal‐to‐vertical ratio check, have also been added to E3 to improve the quality of triggers that are used to create events. Because of its excellent performance, E3 is now the basis for EPIC, the only ShakeAlert point‐source algorithm going forward. We can therefore also use the performance of E3 described here to assess the likely performance of ShakeAlert in the coming public rollout. We should expect false events with magnitudes between M 5 and 6 less than once per year. False events with M6 will be even less frequent, with none having been observed in testing. We do not expect to miss any M6 onshore earthquakes, though the system may miss some large offshore events and may miss one onshore earthquake between M 5 and 6 per year. Finally, in the metropolitan regions where the station density is on the order of 10 km, we expect users 20, 30, and 40 km from an earthquake epicenter to get 3, 6, and 9 s warning, respectively, before the S‐wave shaking begins.

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